<!DOCTYPE HTML>
<html>
  <head>
    <meta charset="utf-8">
    <title>Correctly masked page</title>
    <style>
.box {
  position: relative;
  width: 420px;
  height: 420px;
}
    </style>
  </head>
  <body>
    <div class="box" style="background-color: black;">
    </div>
    <div class="box" style="background-color: white;">
    </div>
  </body>
</html>

